Contemporary fluid dispense systems are well suited for dispensing precise amounts of fluid material at precise positions on a substrate. A pump transports the fluid to a dispense tip, also referred to as a xe2x80x9cpinxe2x80x9d or xe2x80x9cneedlexe2x80x9d, which is positioned over the substrate by a micropositioner, thereby providing patterns of fluid on the substrate as needed. As an example application, dispense tips can be utilized for depositing precise volumes of adhesives, for example, glue, resin, or paste, during a circuit board assembly process, in the form of dots for high-speed applications, or in the form of lines for providing underfill or encapsulation.
FIG. 1 is a perspective view of a conventional dispense tip 24. The dispense tip 24 includes a body 26 and a hollow neck 28. The body 26 attaches to a pump 22, for example by means of a thread, which controls the amount of fluid to be dispensed. The neck 28 is typically a hollow cylinder having a first end 31 which is positioned to overlap with an aperture formed in the body 26, and a second end 30 at which the fluid is dispensed.
As shown in the close-up perspective view of FIG. 2, the neck 28 is formed by rolling a flat portion of machined metal into a cylindrical form. A seam 40 is welded along the longitudinal axis, to seal the edges of the flat portion, using conventional seam welding techniques. In precision tips, the inner diameter of the opening at the second end 30 may be on the order of 0.030 inches in diameter. The thickness of the walls 32 may be on the order of 0.010 inches. A hole 29 is bored into the tip body 26, and the neck 28 is aligned with, and pressed into, the hole. As a consequence of rolling and welding, the inner diameter of the neck is often unpredictable due to inner collapse.
When fluid is released at the opening 30, a high degree of surface tension on the substrate is desired, such that the substrate receives and pulls the fluid from the tip 24. It is further desirable to minimize the surface tension of the neck 28 interface such that when the pin retracts from the substrate, dispensed fluid properly remains on the board. However, a certain degree of surface tension in the neck exists due to the thickness of the walls 32 of the neck 28 at the opening 30.
It has been observed that the surface tension, or xe2x80x9clandxe2x80x9d, at the opening 30 of the neck 28 can be reduced by tapering the outer diameter of the neck 28 to a sharp point. As shown in FIG. 3, the distal end 30 of the neck 28 is sharpened using a surface grinder 42. The neck 28 is positioned perpendicular to the motion of the grinder 42 as shown, to thereby generate a taper 36, or bevel, on the distal end of the neck 28. The tapered portion 36 varies in thickness from the outer diameter of the neck 28 at position 37A to a sharpened point 37B at the opening 30. For the example given above, by providing a taper 36, the amount of land at the opening may be reduced from 0.010xe2x80x3 of contact about the perimeter of the opening, to 0.001xe2x80x3 of contact. In this manner, the surface tension at the junction of the pin and fluid is highly reduced, leading to a higher degree of dispensing precision.
As shown in the close-up perspective view of FIG. 4, as a consequence of formation of the taper 36 in the manner described above, with the neck 28 positioned substantially perpendicular to the grinding wheel 42, tooling scars, in the form of radial rings 38, can form on the taper 36 due to surface variations in the grinding wheel 42. These rings 38 provide ledges or shelves that can lead to additional surface tension on the taper 36, which, in turn, capture fluid material when the tip is released from the substrate following a fluid deposit. This, in turn, can cause fluid to be dispensed inconsistently on the substrate during subsequent deposits, leading to inaccurate results.
The present invention is directed to a tapered dispense tip grinding method, and a dispense tip processed according to such a method, that overcome the aforementioned limitations associated with conventional techniques. In the present invention, the tip is presented to the grinding wheel in a longitudinal orientation - the longitudinal axis of the neck of the tip is substantially aligned with the direction of movement of the grinding wheel. In this manner, the taper is formed without the radial rings of conventional techniques, thereby providing a tip with further-reduced surface tension and therefore increased dispensing precision capability.
In a second aspect, the present invention is directed to an electropolishing technique whereby a beveled tip is electropolished to further buff, or remove, tool marks generated during bevel formation. In this manner, burrs and pits are removed from the surfaces of the tip. This aspect is applicable to treatment of both conventional laterally-ground and the inventive longitudinally-ground tapered tips. Electroplating may further be applied to external and internal tip surfaces to enhance surface lubricity.
In a third aspect, the present invention is directed to a dispense tip formed in a solid unitary piece, machined from stock. By machining the neck opening, potential inner collapse of the neck due to rolling as in prior configurations is avoided. Furthermore, alignment of the neck with the body of the tip is unnecessary and complicated assembly procedures are thereby avoided. The unitary tips further offer the advantage of a robust neck, avoiding the need for bonding of the neck to an alignment foot. Because of the added robustness, the unitary tips are more amenable to deployment with longer-length necks than conventional configurations.
In a preferred embodiment of the third aspect, the neck is of a first inner diameter along a majority of its length, and of a second inner diameter proximal to the opening, the first inner diameter being greater than the second inner diameter. This configuration allows for delivery of the dispensed fluid to the opening at a relatively low pressure, as compared to conventional tips having a single, narrow diameter over their lengths, and is especially attractive to dispensing applications that require smaller diameter tips.
A preferred embodiment of the third aspect of the present invention comprises a unitary fluid dispense tip. The tip includes an elongated cylindrical neck having a longitudinal axis. A bore is machined in the neck centered at the longitudinal axis, the bore having an input end and an output end. The input end of the bore has an inner surface of a first inner diameter and the output end of the bore has an inner surface of a second inner diameter, the first inner diameter being greater than the second inner diameter. An inner taper is machined in the bore such that the inner surface of the bore transitions gradually from the first inner diameter to the second inner diameter.
The inner taper is preferably proximal to the output end of the neck, and is preferably formed at an angle of approximately 20-40 degrees relative to the longitudinal axis of the neck. The neck is preferably formed with a body about the input end of the neck, the body including a funnel adapted for delivering fluid to the input end of the neck. The body may optionally be formed separately from the neck, in which case the body and neck are preferably coupled via press-fitting, bonding, or welding. An alignment foot may be coupled to the body so as to provide a vertical gap below the neck during a dispensing operation. Multiple necks may be mounted to the body, in which case the funnel is adapted for delivering fluid to the multiple input ends of the multiple necks.
A liner sleeve may be inserted in the neck of the dispense tip in order to reduce material flow for low-viscosity materials. The sleeve may comprise, for example, Teflon(trademark) tubing, inserted by a sleeve insertion tool adapted to push the tubing into the neck, and removed by a sleeve removal tool.
In a fourth aspect, the present invention is directed to a cleaning tool adapted for cleaning the inner surfaces of the neck of the dispense tip. The cleaning tool includes an elongated body that serves as a handle during a cleaning operation, and a sharpened shovel adapted to interface with, and shaped to correspond with, the tapered inner diameter of the tip neck. The shovel is located on a bevel, the bevel having an angle substantially similar to the neck taper to allow the shovel to access the tapered portion of the neck. Optional drill flutes may be formed on the cleaning tool body for removing a bulk of the material from the inner surface during a cleaning operation. In this manner, buildup of hardened material is avoided, and dispense tip lifetime is extended.
In a fifth aspect, the present invention is further directed to a cleaning kit for cleaning dispense tips configured in accordance with the present invention, thereby extending the useful lifetime of the dispense tips. The kit is preferably enclosed in a plastic, non-scratch compartmentalized receptacle, and includes a pin-vise, magnet, syringe and plunger, magnifying glass, cleaning wires, and cleaning tools. The pin vise is adapted to secure the miniature wires and drills during a cleaning operation. The magnet is helpful for locating the wires and drills on a work surface, for example by using a sweeping motion of the magnet over the surface. The syringe and plunger are provided for flushing out the dispense tips following cleaning with the wires and fluted drill bits. Alcohol is a preferred liquid for the flushing operation. A magnifying glass helps with inspection of the dispense tips during, and following, cleaning. Cleaning wires include cleaning wires with tapered ends for eased insertion into the dispense tips. Cleaning tools include fluted drill bits for coarse cleaning of the inner necks, a shoveled cleaning tool, described above, for cleaning the inner taper of unitary dispense tips, and a liner insertion tool, described above, for inserting liners into the unitary dispense tips.